I have an Australian friend who lives in Holland. Her job and her husband are in Holland, her family in Australia. She has had to ask to be allowed to only work for six months of the year, doing a sort of job share, because although her brother and sister both still live in Australia she had never ever imagined that they would both more or less refuse to do much to help out with their parents as they both gradually became ill. Her father is now in a wheelchair and her mother has dementia, although it seems to be progressing fairly slowly at this stage.
This is the third year running that she has spent the entire six months running herself ragged, sorting out care, dentist appointments, cleaners, decorating, sorting out their clothing, buying new appliances and their finances. To say nothing of the cost of the flights and car hire. I shan't go into why her siblings refuse to help, but in an ideal world she would ship them back to Holland with her but of course they won't go. It's fascinating to watch what living all over the world can do to a family, I suppose the only system that ever really worked was when none of us ever moved more than two miles from our immediate families, and we all know how claustrophobic that could feel. the payoff however, was the care provided when needed.
I feel for you, having to make such a choice. I really don't know what I would do. Your SiL could skip off to Hong Kong in a few years' time though. Anywhere, really. He has to go where the work is. I do hope you get on well with him.